Quantum efficiency enhancement in multi-junction solar cells with spectrally selective and conducting 1D photonic crystals

Abstract

Herein quantum efficiency enhancements are demonstrated in multi-junction solar cells with one-dimensional Selectively Transparent and Conducting Photonic Crystal (STCPC) intermediate Bragg reflectors. Specifically, the modelling, design and fabrication of series-connected multi-junction cells comprising a top amorphous-silicon cell and a bottom crystalline-silicon heterojunction cell with spectrally tunable STCPC intermediate reflectors (IR) is presented. Wave-optics modeling analysis shows current density gains of up to 17.4% and 10.4% compared to multi-junction cells without an IR and with an industry standard IR comprising a ZnO film, respectively. Fabricated multi-junction cells based on these modelled structures demonstrate that the STCPC IR enhances cell currents by 17.1% and 13.2% compared to the reference cases without an IR and with a ZnO film IR, respectively. From an efficiency standpoint, the STCPC IR improved cell performance by 7.9% over reference cells without an IR, but was 8.9% less efficient relative to the reference cells with a ZnO IR. The lower overall efficiency of the tandem cell with the STCPC IR with respect to that of the cell with the ZnO IR is primarily due to non-optimal electrical performance of the STCPC compared to the single ZnO layer. Methods for improving the electrical properties are discussed.